LTE eNodeB Scheduler and Different Scheduler Type

What is a LTE Scheduler ?

Scheduling is a process through which eNodeB decides which UEs should be given resources (RBs), how much resource (RBs) should be given to send or receive data .In LTE, scheduling is done at per subframe basis i.e. every 1 mili second. The entity which is govern this is know as scheduler.

Some or all of these characteristics determine what kind of Packet schedulers are required at the LTE MAC to adhere to the required QoS requirements of the relevant applications. LTE MAC supports the following three types of Scheduling:

Dynamic Scheduling

Persistent Scheduling

Semi-Persistent Scheduling

Dynamic Scheduling: Every TTI, MAC checks for the UEs to be scheduled, the Data Availability for each UE to be scheduled and the feedback from the UE on the Channel conditions. Based on these data, it can schedule the resources for the UE through the PDCCH. If data is not available, UE will not get scheduled. All Services can be scheduled using Dynamic Scheduling, but at the expense of the Control signalling [PDCCH Usage – a scarce resource].

Persistent Scheduling: In this case, Packets are scheduled on a fixed basis, similar to the Circuit Switched fashion. Here, it does not depend on the Channel Condition. The Resource allocation remains constant for the period of the call.

Semi-Persistent Scheduling: It is a Hybrid way of scheduling, which tries to overcome the drawbacks of the Dynamic Scheduling and the Persistent Scheduling.

Semi-Persistent Scheduling

The figure above show conversational voice is considered for persistent scheduling. It is clear that, because of the fixed resource allocation, UE will end up in under utilizing the allocated resources, because of non-availability of the sufficient data during the persistently scheduled TTIs. Here, because the Speaker sometimes speaks and sometimes user may give pauses during the conversation, Voice activity period and Voice Inactivity period exist in the Speech data. If the Speech Codec without VAD is involved, then it sends the Voice Payload at the end of every voice frame length of 20ms during the Voice activity period and sends nothing during the Voice Inactivity period. So, the fixed resource allocations will get utilized during the Voice activity periods to transmit the received voice payloads over the air interface and it gets unused during the Voice inactivity periods, which is a critical drawback. Semi persistent scheduling addresses this drawback in a unique way.

As shown above, whenever the Voice Payloads arrive at the L2, the MAC Scheduler will activate the SPS resources and whenever there are no transmissions for few of the transmission opportunities, the SPS resources were implicitly released. Again, it gets activated, when the voice payloads arrives at the next Voice activity period. During the Voice Inactivity period and after the implicit release of the SPS resources, these radio resources will be allocated for different UEs, which are in need of it. It is clear that, only those services, which are real time in nature, with fixed packet size payloads, and fixed periodicity of the payload arrival, can effectively and efficiently utilize. The Semi-Persistent Scheduling. Such services or applications, which fulfils these characteristics are conversational voice, conversational video [only Conversational voice part of the video], and any other real time applications [Only Conversational Voice part]

Different Types of Schedulers:

Round Robin: The RR scheduler selects and schedules UEs in a round robin manner, thereby creating an equal resource share. The disadvantage of this approach is that UEs with sub-optimalCQIs may be allocated Physical Radio Resources (PRBs), thus reducing the overall cell throughput.

Max CQI : The max-CQI scheduler selects the schedulable UEs based on the experienced CQI. The UEs with the highest CQI therefore become candidates for scheduling thereby increasing the overall cell throughput. The disadvantage of this approach is that UEs with lower CQI are denied scheduling instances, thus being starved for throughput and leading to degraded user experience.

Proportional Fair: The PFS is expected to strike a balance between the traditional Round Robin (RR) scheduler and the max Throughput Scheduler (also known max-CQI (Channel Quality Indicator) scheduler). The PFS scheduler performs in such a manner that it considers resource fairness as well as maximizing cell throughput (in addition to other possible
performance metrics).

For a Max C/I scheduler, the Sector throughput improves while cell edge throughput drops compared to a PF scheduler where sector throughput may not be as good as Max C/I but cell edge throughput thoroughly improves.

Scheduler Type

Max C/I

Round Robin

Proportional Fair (PF)

How it works

Allocates resources to the
user with the instantaneous best RF conditions. UE with the best channel conditions is always prioritized

Resources are shared
across users over time
regardless of the RF conditions.

Sharing the cell throughput but as a function of RF conditions and bearer
priorities